Hot compaction is a process which is well-known for use with semi-crystalline and crystalline polymer materials such as polypropylene and the like and is disclosed in U.S. Pat. No. 5,628,946. During a heating step, a small portion of the surface of the polymer strand melts and during subsequent cooling re-crystallises in a different form from that of the initial material in a manner which facilitates bonding of the fibres to each other. Typically, several layers of the material are placed in a mould and then heated and pressed at a predetermined pressure for a predetermined period of time before being cooled and then removed from the mould. The specific temperatures, pressures and dwell times associated with the hot-compaction of particular polymer materials are normally determined by experimentation for each individual product. However, in each case the temperature pressure and dwell time will be sufficient to allow melting and inter-bonding of the outer layers of the polymer strands without destroying the properties of the core of each strand and the pressure will be sufficient to cause plastic flow of any softened material so as to encourage good bonding between adjacent polymer strands. It is important to appreciate that one should not over heat the polymer material as to do so might result in a final product which is of less than optimal performance.
The production of polymer articles by hot-compaction has, until now, been confined to crystalline or semi-crystalline polymers as such polymers have easily defined and distinct melting temperatures and it is, therefore, possible to accurately define a temperature and dwell time during which the outer surface of the polymer strands will melt and bond to each other whilst the inner core remains unaltered. Amorphous polymer materials do not have a specific and easily defined melting temperature but have what is known as a “glass temperature” “Tg” which is a temperature at and above which the polymer progressively softens and becomes more pliable without actually melting in the manner of crystalline or semi-crystalline materials. The particular glass temperature (Tg) for each amorphous polymer will vary depending on the particular material and also on the temperature of polymerisation.